Method for coating tortuous aluminum shapes



No. 593,744 Claims. (Cl. 148-6.16)

ABSTRACT OF THE DISCLOSURE This invention relates to the art of coating aluminum, especially tortuous aluminum shapes, with an acidic chemical conversion coating to impart good corrosion resistance and paint adhesion properties to the aluminum surfaces.

The present invention involves the use of acidic coating solutions having as their active ingredients chromic acid, fluoride, and phosphoric acid. Solutions of this general type have been known in the art for some time; however, it has been found that they and other related solutions are often economically unattractive for use on tortuous aluminum shapes because the high drag out inherent in the processing of such shapes causes great wastage of material. It has been thought heretofore that certain minimum concentration of the above coating ingredients are necessary to obtain a suitable coating, or, in fact, to obtain a coating to all. The invention is based on the discovery that successful and useful conversion coatings can be obtained by using solutions far more dilute than those heretofore used. According to the invention the concentrations of fluoride and chromic acid may each be as low as 0.1 g./l. and the concentration of phosphoric acid may be as low as 1 ml./l. calculated as 75% phosphoric acid. This discovery has made it possible, as a practical matter, to obtain suitable conversion coatings on tortuous aluminum shapes at reasonable costs.

As pointed out above, it has been known for some time that protective chemical conversion coatings can be placed on aluminum by the use of solutions containing chromate, fluoride, and phosphate. Various solutions of this general type are disclosed in the following US. Patents: 2,438,877; 2,472,864; 2,494,910; 2,678,291; and 2,928,763. While these patent disclosures singly, and collectively, indicate that some sort of coating on aluminum can be obtained at fairly dilute concentration of active ingredients, they all contain caveats concerning the difficulties which will be encountered at the lower end of the operative ranges given in the patents, and they all set preferred ranges of concentrations considerably above their stated lower limits. Commercial practice in the several years since the issuance of the above patents has been substantially confined to the use of solutions containing the active ingredients within or near the preferred concentration ranges set out in the patents. In general, it has been thought that the operating problems attendant on the use of such coating solutions are minimized in commercial practice by using such relatively concentrated solutions.

Such relatively concentrated aluminum treating solutions have proved satisfactory when the aluminum shapes being treated are generally flat; a typical example is aluminum coil stock. However, recently aluminum forming technology has advanced so that it is practical to construct complex or tortuous shapes of aluminum. For example, aluminum tin cans are now made for food stuffs, beverages, and other liquids. Another recent example of a tortuous aluminum assembly is an automobile 70 radiator. In most cases an aluminum article as complex as an automobile radiator will be made up of several parts which are brazed together and will have brazing flux residue on it resulting from the brazing.

Whatever the specific shape of the tortuous aluminum articles, they present a common problem to one attempting to apply a chemical conversion coating to them. Because of their tortuous shape, they tend to drag out from the coating stage of an assembly line much more coating solution than do relatively fiat or simple aluminum articles. This problem occurs whether the treating stage is of the spray type or immersion type. In the case of an aluminum can, it can often happen that a particular can will leave the treating stage brim full of coating solution. Furthermore, the normal drag out minimizing procedures which are effective with flat aluminum stock are largely inapplicable to tortuous shapes. Thus, the squeegee rolls or wiper arms commonly used with aluminum coil stock cannot be effectively used with a can or an automobile radiator. When the relatively concentrated conversion coating solutions of the prior art are used on such shapes it is found that the inherently high volume of drag out causes economic loss of expensive chemicals, and complicates further processing operations, such as rinsing, etc. This factor alone has presented a serious barrier to the effective use of the conversion coatings of the prior art on tortuous aluminum shapes.

Aluminum cans intended for use as food stuff containers or beverage containers present further problems. Conversion coatings produced by the solutions of the patents mentioned above are, in the first instance, desirable for use on food and beverage cans because such coatings are free of hexavalent chromium. As is known, hexavalent chromium is toxic and can not be used on metals coming in contact with food. (For this reason, other kinds of chemical conversion coatings for aluminum are unusable on food containers as a practical matter.) However, as the cited patents point out, it is in most cases desirable, and sometimes essential, to follow the prior art coatings by a dilute rinse of chromic acid.

This rinse often results in the incorporation of some,

hexavalent chromium into the coating, and the advantage of this type of coating over others for food stuff containers is thereby largely dissipated. Furthermore, it should be noted that food stuff containers are often painted or lithographed, and it is desirable that the conversion coating on the metal be as colorless as is practical so it will not interfere with such decorative labels. The relatively concentrated solutions of the prior art characteristically produce a color which is greenish in hue.

The tortuous aluminum assemblies which are multipart present still another problem. The parts are quite often attached to one another by brazing techniques and there inevitably results a residue of brazing flux on the assembled article. This flux must be completely removed before a chemical conversion coating can be applied, and the coating materials of the cited patents are rather ineflicient as flux removers. Thus when a brazed tortuous aluminum article is being processed, an additional pre-treatment stage is necessary to remove the brazing flux.

I have discovered that the foregoing problems can in large measure be eliminated by theuse of chemical conversion coating solutions containing chromate, fluoride, and phosphate at concentrations far more dilute than those heretofore thought possible, when the solutions are specially formulated so that the chromate is derived from chromic acid, the phosphate from phosphoric acid, and the fluoride from hydrofluoric acid or a simple salt thereof.

It is an object of this invention to provide an improved method for applying a protective conversion coating to tortuously shaped aluminum articles which greatly reduces the economic loss resulting from drag out.

A further object of this invention is the provision of a method for applying chemical conversion coatings to tortuous aluminum surfaces, which coatings do not require an acidulated rinse of dilute chromic acid.

Still another object of this invention is the provision of an improved method for applying a chemical conversion coating to multipart brazed aluminum articles, in which method the brazing flux residue is removed from the article by the coating solution during the course of formation of the conversion coating.

Other objects will appear from the following detailed description.

I have discovered that aluminum surfaces, especially tortuous surfaces, can successfully be treated to apply a good quality conversion coating by subjecting them to the action of a dilute aqueous coating solution, the coating producing ingredients of which are chromic acid, phosphoric acid, and a fluoride selected from the class consisting of hydrofluoric acid and simple salts thereof. I have found that these three materials may be used successfully in much lower concentrations than those contemplated or taught by the prior art. In this connection, it should be noted that certain of the prior art patents suggest that the chromate and phosphate ingredients be supplied as salts. I have found, however, that at the high dilutions at which my invention is operated, these materials should be supplied as the acids. The concentration of chromic acid may be as low as 0.10 g./l. (calculated as CrO and the concentration of phosphoric acid can also be very low, even as little as 1 ml./l'. (calculated as 75% phosphoric acid). The ponent, which is preferably supplied as hydrofluoric acid, can be as low as 0.1 g./l. (calculated as HF). With solutions of the acidic character of those employed in accordance With the invention, aluminum will dissolve in some measure from the surfaces being treated, and as a particular treating bath is used, the concentration of aluminum in it will build up over the course of time. The dissolved aluminum ties up fluoride as aluminum fluoride, and the fluoride so tied up is substantially unavailable as an active coating producing ingredient. The amount of fluoride which is eflectively lost in this manner amounts to three mols for every mol of dissolved aluminum. It should be understood that when the term fluoride concentration is used in this disclosure and in the claims, it is intended to refer to the available or active fluoride rather than the total fluoride. The level of active fluoride may be measured in accordance with US. Patent 2,814,557 (involving measurement of active fluoride by its ability to etch glass), and U.S. Patent 3,129,148 (an instrumental method), or in accordance with other known methods. At coating ingredient concentrations below the limits set out above, no useful coatings are obtained.

The fluoride component is preferably supplied to the solutions used in accordance with the invention as hydrofluoric acid, which has the advantage of obviating the introduction of extraneous cations in the solutions. However, if it is desired, simple fluoride salts, such as sodium fluoride, may be used as the source of the fluoride component. Except for the cation which is associated with such an alternate source of fluoride, I generally prefer to avoid the presence of extraneous anions and cations in the solutions of the invention, although small concentrations of other materials may be tolerated.

The specification of upper limits for the concentration of the materials forming the coating ingredients is somewhat diflicult, since one can, if he desires, use the relatively concentrated solutions taught by the prior art patents cited above, these solutions being, of course, operative. However, I have found that the advantages of my invention are lost, or dissipated, if the concentrations of the coating ingredients exceed certain levels. That is to say, while conversion coatingcan be obtained at higher conconcentration of the fluoride comcentrations, the critical advantages of minimizing uu economical drag out loss, eliminating altogether the need for a dilute chromic acid rinse, and eliminating the need for a brazing flux rem-oval stage in the case of multi-part articles, are lost. Thus, the upper limits on coating ingredients concentrations for my invention in view of these considerations are as follows. The chromic acid concentration should be less than about 2.4 gms./l. (calculated as CrO The phosphoric acid concentration should be less than about 15 mls./l. (calculated as 75% phosphoric acid). The concentration of fluoride, whether supplied as the acid or a simple salt, should be less than about 1.5 gms./l. (calculated as HF).

There is presented below a table illustrating a series of coating baths operated in accordance with the invention. The table shows the composition of each such bath and the weight of coating obtained. The anions were supplied to the baths as chromic, hydrofluoric, and phosphoric acids. All of the, aluminum surfaces treated in these baths were subjected to a 30 second spray with a solution temperature of F.

Coating Bath Ct'Oa, gJl. P04, g./1. HF, g./1. HF/CrOs Weight, No. rug/sq. it.

All of the coatings from the above listed baths were of good quality and appearance and gave satisfactory corrosion resistance protection to the underlying metal. Furthermore, all of the coatings provided good paint bases and improved the paint adhesion properties of the metal. The light-weight coatings were almost colorless.

From the above table it can be seen even though the coating producing ingredients are used at very high dilution, a wide variety of coating weights can be obtained, ranging from a few milligrams/ sq. ft. to nearly 200 mgs./ sq. ft. It can also be seen from the table that with the phosphate and fluoride concentrations held constant, an increase in the chromate concentration produces an increase in the coating weight. Stated differently, when the phosphate concentration is held constant, a decrease in the fluoride/chromate ratio results in an increase in coating Weight.

As has been discussed above, the coatings produced in accordance with this invention do not need a dilute final rinse of chromic acid. They need merely be rinsed with water which is preferably substantially ion free, such as deionized water or distilled water.

Furthermore, as was pointed out above, the dilute coating baths of the invention successfully dissolve any brazing flux residue on the aluminum surface. In this connection, I have found that if an ultrasonic field of a frequency of about twenty kilocycles is applied to a solution formulated in accordance with the invention during the time that a brazed article is immersed in it, the time for removal of the flux can be materially reduced, for example from 25 to 30 minutes to about 5 minutes. This is of substantial advantage since the brazing flux removal operation can thus be accomplished in a time period which is of the same order of magnitude as the time period which is appropriate for application of the conversion coating. Since 'both operations occur at once, a single 5 minute immersion of the article both removes the flux and applied the conversion coating.

Finally, I have found that the economic savings of chemicals flowing from the use of the dilute solutions of the invention on tortuous aluminum articles are so great that it is for the first time economically feasible to form coatings of this type on such articles, and thereby gain the several advantages which these coatings provide.

I claim:

1. A method for applying a protective coating to the surface of tortuously shaped aluminum articles under conditions where there is a high drag out of coating solution to impart good corrosion resistance and paint adhesion properties to the surface, comprising subjecting said surface to the action of an aqueous coating solution, the essential coating producing ingredients of which consist essentially of chromic acid, phosphoric acid, and hydrofluoric acid, in coating producing proportions, said ingredients being in proportions selected to minimize drag out losses of solution and base metal color change resulting from the action of said coating solution, said chromic acid being present in said solution in an amount between 0.1 and 2.4 g./l., calculated as CrO said phosphoric acid being present in an amount between 1 ml./l. and 15 ml./l., calculated as 75% phosphoric acid, and said fluoride being present in an amount between 0.1 and 1.5 g./l., calculated as HF, the source of each of said ingredients being the corresponding acid of the ingredient, and thereafter rinsing said surface with substantially ion-free water.

2. A method for treating a tortuously shaped multipart aluminum article having brazing flux residue thereon to simultaneously remove said brazing flux residue from the article and apply a protective coating to the surface thereof comprising immersing said article in an aqueous solution the essential active flux dissolving and coating producing ingredients of which consist essentially of chromic acid, phosphoric acid, and a fluoride selected from the class consisting of hydrofluoric acid and simple salts thereof, said chromic acid being present in said solution in an amount between 0.1 and 2.4 g./l., calculated as CrO said phosphoric acid being present in an amount between 1 ml./l. and 15 ml./l., calculated as 75% phosphoric acid, and said fluoride being present in an amount between 0.1 and 1.5 g./l., calculated as HF, applying an ultrasonic field of about 20 kilocycles to said solution while said article is immersed therein, whereby the brazing flux residue is removed from the article and a protective coating is applied thereto, and thereafter rinsing said surface with substantially ion-free water.

3. A method for applying a protective coating to aluminum surfaces, under conditions where inherent high drag out of coating solution is experienced, to impart good corrosion resistance and paint adhesion properties to the surfaces, comprising subjecting said surfaces to the action of an aqueous coating solution, the essential coating producing ingredients of which consist essentially of chromic acid, phosphoric acid, and hydrofluoric acid, in coating producing proportions, said ingredients being in proportions selected to minimize drag out losses of solution and base metal color change resulting from the action of said coating solution, said chromic acid being present in said solution in an amount between 0.1 and 2.4 g./l., calculated as CrO said phosphoric acid being present in an amount between 1 ml./l. and 15 ml./l., calculated as phosphoric acid, and said fluoride being present in an amount between 0.1 and 1.5 g./l., calculated as HF, the source of each of said ingredients being the corresponding acid of the ingredient, and withdrawing said surfaces from said coating solution under conditions which cause high drag out of said coating solution.

4. The method according to claim 3 wherein the coated surfaces are rinsed with substantially ion-free water.

5. The method according to claim 3 wherein at least part of the aluminum surfaces are tortuously shaped.

References Cited UNITED STATES PATENTS 2,438,877 3/1948 Spruance.

2,472,864 6/1949 Spruance et al. 2,494,910 1/ 1950 Spruance et al. 2,678,291 5/1954 Spruance et al. 2,928,763 3/ 1960 Russell et al.

. FOREIGN PATENTS 891,910 3/1962 Great Britain.

RALPH S. KENDALL, Primary Examiner. 

